DYNAMICS The study of why objects move or dont move.

DYNAMICS

The study of why objects move or don’t move.

What is the natural state of motion of an object?

Aristotle Observed:

• All objects eventually come to a stop.

• Therefore he concluded the natural state of motion for an object, is for the object to be at rest.

• He Incorrectly concluded that the only way an object stays in motion is if it has a force acting on it.

• But, what is a force?

Force – A push or a pull caused by the interaction of two objects.

• F = Force

• Units: Newtons (N) or pounds (lb)

• 1lb = 4.45N

• A force is a vector quantity – It has magnitude (value) and direction.

Weight is a Force

• It is caused by the interaction between the earth and an object.

• It has a value and a direction (down).

• Objects have the same mass everywhere, they only have weight when they interact with a planet, like earth.

• A 150lb person has a weight of 668N.

Copernicus Observed:

• The motion of the stars and planets.

• He concluded that the earth was in constant motion.

• This idea was highly controversial.

• People preferred to think the earth was the center of the universe.

• As a result he was persecuted and had to work in secret.

Galileo devised experiments to discover the natural state of

motion.

• His experiments represent the birth of modern day science.

• He also suffered persecution for his efforts.

Galileo’s Experiments:

A ball rolling down a ramp, rolls to the same height on a ramp across from it. He worked very hard to eliminate friction, but of course he never could completely.

ho hf

ho=hf

He changed the angle of the second ramp and observed the ball rolled further before reaching hf.

ho=hf

ho hf

He than imagined what would happen if the second ramp was flat. Hmmm? What would happen? (remember to neglect friction)

The ball is trying to roll back to a place where ho = hf

ho hf

The ball would roll forever with a constant velocity!!!!!!

Galileo’s experiments demolished the idea that a

force is necessary to keep an object moving.

The natural state of motion for an object is to be moving with a constant velocity, which could

be zero.

So an object at rest is just a specific example of an object with a constant

velocity.

Sir Isaac Newton was the next major player on the scene

• He was born on Christmas day, the year Galileo died.

• His work influenced today’s world at a monumental level.

• He developed three laws that describe the motion of everything.

• He very well may have been one of the most influential human beings to ever live.

• And you thought he was just some dumb guy who got hit on the head with an apple.

1642-1727

Inertia

• The natural tendency of an object to resist a change in motion.

• Mass is a measure of inertia.

Newtons 1st Law:

An object with constant velocity (which could be zero) continues with constant velocity (same speed and direction) unless an unbalanced force acts on it.

Newtons 2nd Law:

The sum of the forces on an object is equal to the mass times the acceleration of the object.

maF The sum of

Newtons 3rd Law:

For every force (action), there is an equal and opposite force (reaction).